Various thermal transfer methods are known in the art, and one of them is a sublimation type thermal recording system in which, using a thermal transfer sheet comprising a substrate film bearing a dye layer containing a heat sublimable dye and a binder, the dye contained in the dye layer is sublimated (thermally transferred) in response to image information by heating means, such as a thermal head or a laser beam, to conduct recording. Further, a hot-melt type thermal recording system is also known in which, using a thermal transfer sheet comprising a substrate film bearing a hot-melt ink layer containing a colorant, such as a pigment, and a vehicle, such as wax, a hot-melt ink component softened by the same heating means as used in the sublimation type thermal recording system is transferred to form an image. These thermal transfer methods permit various images to be simply formed and, hence, have become utilized in prints wherein the number thereof may be small, for example, in the preparation of identification (ID) cards and various certificates.
For the preparation of the ID card or the like using the above thermal transfer sheet, the use of the hot-melt type thermal transfer sheet can easily provide monotonous images, such as letters and numerals. These images, however, are disadvantageously poor in fastness, particularly abrasion resistance. On the other hand, the use of the sublimation type thermal transfer sheet enables a gradational image, such as a photograph-like image of a face, to be precisely formed. Unlike usual images formed using a printing ink, however, the image formed by the sublimation type thermal transfer sheet is free from any vehicle and is unsatisfactory in fastness, such as weather resistance, abrasion resistance, and chemical resistance, although the image is a dye image.
Lamination of a thermal transfer film, for a protective layer, having a thermally transferable resin layer onto an image formed by the thermal transfer of a hot-melt colorant layer or a heat sublimable dye followed by the transfer of the thermally transferable resin layer by means of a thermal head or a heating roll to form a protective layer on the image is known as means for solving the above problem.
The provision of the protective layer can improve the abrasion resistance, chemical resistance, solvent resistance and other properties of the image to some extent, and the incorporation of an ultraviolet absorber or the like to the protective layer results in improved light fastness. Since, however, the protective layer has no water absorption capacity, an image with the above protective layer cannot be used in applications where stamping in an aqueous ink is necessary, for example, for a photographic image of a face in a passport. Therefore, this unfavorably imposes a limitation on the type of writing materials and stamps usable in writing of letters or affix of stamps.
Further, there is a demand for higher chemical resistance and solvent resistance of a photographic image of a face for a passport or the like from the viewpoint of preventing forgery of the passport or the like. Conventional protective layers, even those formed of a cured or crosslinked resin, however, offer no satisfactory fastness. Accordingly, an object of the present invention is to solve the problems of the prior art and to provide a thermal transfer film, for a protective layer, which, upon transfer on a print, can form a protective layer, for an image, having a capability of absorbing and fixing an aqueous ink and, hence, possessing excellent stampability and writability and, at the same time, possessing better chemical resistance and solvent resistance, and to provide a print with the above protective layer formed thereon.